For example, in an advanced device that adopts a silicon wafer having a larger diameter typified by, e.g., a diameter of 300 mm, a magnitude of a surface waviness component called a nanotopography has recently become a problem. The nanotopography is one kind of surface shapes of wafers, indicative of irregularities of a wavelength component of 0.2 to 20 mm in which a wavelength is shorter than that of Sori or Warp but longer than that of surface roughness, and is a very shallow waviness component having a PV value of 0.1 to 0.2%. It is said that this nanotopography affects a yield ratio of an STI (Shallow Trench Isolation) process in a device process, and a rigorous level and fineness of a design rule are demanded with respect to a silicon wafer that becomes a device substrate.
The nanotopography is created in a machining process of a silicon wafer. It is apt to be degraded particularly in a processing method having no reference surface, e.g., wire saw cutting or double-disc grinding, and an improvement or management of relative meandering of a wire in the wire saw cutting or a warp of the wafer in the double-disc grinding is important.
The nanotopography of a silicon wafer after mirror polishing is generally measured by an optical interferometer Nanomapper (manufactured by ADE Corp.) or Dynasearch (manufactured by Raytex Corporation).
FIG. 9 are nanotopography maps measured by Nanomapper and show intensities of nanotopography by shading. FIG. 9(a) shows an example of a map that does not have a problem in an intensity level of the nanotopography in particular, and FIG. 9(b) shows an example of a bad level created in a double-disc grinding process.
When a workpiece in a process such as a slicing process or double-disc grinding process is a non-mirror workpiece, performing arithmetic bandpass filter processing with respect to a Sori shape obtained from a measuring instrument adopting a capacitance system enables measuring a nanotopography in a simplified manner as disclosed in International Publication No. 2006/018961.
FIG. 10(a) shows an example of a pseudo-nanotopography obtained by performing bandpass filter processing of 50 mm to 1 mm with respect to a Sori shape of a double-disc ground wafer measured by a measuring instrument adopting the capacitance system. It is to be noted that FIG. 10(b) is a graph showing a nanotopography when measured by Nanomapper.
To satisfy conditions under which a nanotopography level having a wavelength size of 10 mm is not greater than 15 nm in a final product, which is becoming a mainstream as a recent demand, a pseudo-nanotopography must be equal to or below 0.2 μm in an intermediate process.
FIG. 12 shows a relationship between a value of a pseudo-nanotopography after the double-disc grinding process and a value of a nanotopography after a final process. It can be understood that they have a good correlation.
A conventional double-disc grinding method will now be described.
First, FIG. 8 shows an example of a conventional workpiece double-disc grinding apparatus that is used for double-disc grinding. As shown in FIG. 8, a double-disc grinding apparatus 101 includes a workpiece holder 102 that supports a workpiece W having a thin plate-like shape from an outer peripheral side along a radial direction and can rotate, a pair of static pressure support members 103 that are positioned on both sides of the workpiece holder 102 and support the workpiece holder 102 from both sides along an axial direction of the rotation in a contactless manner by using a static pressure of a fluid, and a pair of grinding stones 104 that simultaneously grind both surfaces of the workpiece W supported by the workpiece holder 102. The grinding stones 104 are disposed to a motor 105 and can rotate at a high speed.
When such a double-disc grinding apparatus 101 is utilized to grind both surfaces of the workpiece W, the workpiece W is first supported by the workpiece holder 102. It is to be noted that rotating the workpiece holder 102 enables rotating the workpiece W. Further, a fluid is supplied to a space between the workpiece holder 102 and the static pressure support members 103 from the respective static pressure support members 103 on both sides to support the workpiece holder 102 along the axial direction of the rotation by using a static pressure of the fluid. Furthermore, both the surfaces of the workpiece W that is supported by the workpiece holder 102 and the static pressure support members 103 and rotates are ground by using the grinding stones 104 rotating at a high speed by the motor 105.
Various improvements of means for supporting a workpiece in a rotation axis direction have been conventionally examined since a damage of the workpiece produced during grinding affects an accuracy or a nanotopography of a processing target surface.
For example, International Publication No. 2000/67950 suggests performing grinding while controlling a relative position of the center of a thickness of a workpiece and/or the center of supporting means for supporting the workpiece and the center of an interval between grinding stone surfaces of a pair of grinding stones for grinding.
Moreover, for example, in such an apparatus adopting a static pressure support using a fluid as depicted in FIG. 8, Japanese Unexamined Patent Publication (Kokai) No. 2007-96015 discloses that a nanotopography component that cannot be sufficiently improved by an adjustment function provided in the conventional apparatus, i.e., tilt adjustment or shift adjustment of a grinding stone axis can be improved by adopting a static pressure supply member in which each of a plurality of pockets has support holes for a fluid and which can adjust a static pressure of the fluid in accordance with each pocket with respect to a static pressure support method for front and back surfaces that support a workpiece in an axial direction.
As described above, in the conventional technology, preventing the workpiece from being deformed as much as possible during grinding is important in the light of the nanotopography, and energy has been contributed to tilt control or shift control over the grinding stone axis or control over a static pressure that supports a workpiece at an adequate position in the rotation axis direction.
However, when such a conventional double-disc grinding apparatus or double-disc grinding method is utilized to measure a pseudo-nanotopography of a wafer subjected to double-disc grinding, there are many irregularities, and a nanotopography level having a wavelength size of 10 mm exceeds 0.2 μm in some cases. When the pseudo-nanotopography in the double-disc grinding process exceeds 0.2 μm in this manner, a nanotopography level exceeds 15 nm in a final product, and suppressing the nanotopography to a level that has been recently demanded is difficult (FIG. 12).